The present invention generally relates to a telephone communication system and, in particular, relates to one such telephone communication system having a line interface means for establishing communication with a single externally provided pair of telephone wires and means, associated with each of a plurality of telephone subsets, for establishing communication between either another one of the plurality of telephone subset or, via the line interface means, to the single externally provided pair of telephone wires.
In recent years telephone subscribers in the United States and other countries have become entitled to connect their own equipment to externally provided telephone lines. Consequently, a rapid expansion in residential oriented telephone systems and equipment has been developing to satisfy an ever growing number of needs related to such telephone communications.
One particular identified need in such residences is the ability to communicate between and among a plurality of telephone subsets all being interconnected with the single central office telephone line pair of wires that accesses the residence. In addition, other identified needs of subscribers are similar to those of businesses having an independent private automatic branch exchange (PABX). However, the residential subscriber usually does not have the resources to purchase such an exchange. Nevertheless, numerous attempts have been made to provide the PABX type features with systems directed for sale in the residential market. This market, in the field of telephone exchange equipment is unique in requiring equipment that is considerably lower in cost than past similarly functioning equipment as well as requiring relatively simplified installation to allow the subscriber to install the equipment without requiring a representative of a telephone service company.
As it happens, this burgeoning market is not at all limited to residences. For example, many small businesses, such as hardware stores, drug stores, retail outlets and the like, also have need for a communication system having many PABX type features. In addition, a common characteristic in many small businesses, in particular, doctor's offices and other professional offices, is the need to permit multiple user access to a minimal number of telephone lines. The most conventional arrangement is commonly known as a "secretarial system" where a telephone line is, in fact, shared between a supervisor and a secretary. Typically, such secretarial systems permit incoming calls to be screened by the secretary and, in addition, allow intercommunication between the secretary and the supervisor for local communications. Present secretarial systems not only require specialized telephone subsets but, additionally, require special wiring for the intercommunication feature, i.e. usually about 10 to 25 pairs of wires. Although limited versions of telephone communication systems exist, a vast majority of present systems require individual interconnections to a master control switch and thus are disadvantageous by requiring either a rewiring cost upon installation and/or a prohibitive cost penalty for the subsequent inclusion of additional subsets.
In general, consumers of telephone communication systems are confronted by one or more of the following limitations presently existing in conventional systems: that special wiring is required; that the existing telephone subsets are obsoleted; that the cost is excessive in consideration of a residential market; that only limited performance is provided in the event of power failures; that the available PABX type features are limited; and that after installation, further expansion and growth is difficult in time and/or expense.